Friday, March 7, 2014

The Pros and Cons of Mating with an Amazonian Female

By Rachel Wilson

Sex is important, and not just because it produces babies
and facilitates social bonding but because it allows for genetic recombination.
This allows offspring to be genetically unique which ultimately increases
resilience of the species. In turn resilience increases the chances that at
least some individuals in a population will be able to cope and adapt to random change in the environment. This is good, as it maintains biodiversity
and as a biologist this is something that I think is important.

Genetic recombination is
achieved through reproduction. Sexual reproduction results in, arguable, the greatest
genetic diversity as two distinct individuals copulate to produce genetically
unique offspring. This just so happens to be the type humans are most
familiar, mainly because we perform this kind of reproduction. With life being
so very diverse on our planet, there has to be other forms of reproduction out
there! And no, I don't mean sexual fetishes…

I mean reproduction, or, the production of offspring. Some
organisms reproduce asexually, meaning the genetic material inherited by the
offspring is from a single parent. This type of reproduction is called
parthenogenesis. This doesn’t necessarily mean that the offspring is a clone,
although some organisms, mainly microbes, reproduce this way. In fact, some asexually
reproducing species require copulation, or mating, to procreate. However, the
paternal genetic material ultimately is ‘kicked out’ from the embryo. Talk
about being used for sex.

http://m.lmaobruh.com/p/646

Parthenogenesis occurs when the progeny of an individual
only has the maternal genetic material. A subset of parthenogenesis,
gynogenesis ‘kicks out’ the paternal genetic material after mating, and
therefore only the maternal genetic material contributes to the offspring’s
genome. However, unlike parthenogenesis, species reproducing by gynogenesis
must mate with a male (Schlupp et al., 1994). The schematic below sums up this
nicely. M refers to maternal contributions, while P refers to paternal
contributions (Schlupp et al., 1994).

Schlupp et al., 1994 Figure 1

Asexual reproduction implies that all the offspring will be
genetically very similar to their parents. As the population is all female, the
progeny will be as well (usually). But if it’s an all-female species, then how
do they mate with males? It turns out theses females look to their close
relatives to mate. The Amazon molly (Poecilia
Formosa), a gynogen species, mates with either a male Atlantic molly (Pocecilia mexicana) or sailfin molly (Poecilia latipinna) in order to produce
offspring (Marler and Ryan 1997).

Since, the male contributes no genetic material, I presumed there
would be a very strong selection pressure against mating with a female who is
literally using you for sex. Males mating with gynogens aren't increasing their
fitness by having more offspring because the offspring produced aren’t
genetically similar. So I would think that males would preferentially chose not
to mate with Amazon mollies, or they would have to be coerced into mating with such
an Amazon.

Male Atlantic and sailfin mollies preferentially chose to
mate with their corresponding species, although sailfin mollies as less
discriminating than Atlantic mollies (Ryan et al., 1996; Gabor and Ryan, 2001).
Despite this preference, Amazon molly populations still persist. Therefore
there must either be some sort of advantage male Atlantic or sailfin mollies obtain
from mating with these females, or the males somehow mitigate the decrease in
fitness caused by mating with a heterospecific, or a differing species.

In order to mitigate the cost of decreased fitness of mating
with heterospecifics, male sailfin mollies employ two tactics. Firstly, as
seen below, male sailfin mollies spent less time with Amazon mollies during the
sailfin molly breeding season (Heubel and Schlupp, 2008). Correspondingly,
there was a significantly lower proportion of Amazon molly juvenilles in the
wild during the sailfin molly breeding season (Heubel and Schlupp, 2008). These
figures show that when the availability of receptive sailfin molly females
increased, male sailfin mollies ditched their second choice Amazon mollies and
started spending time with females that would give them genetically similar
progeny thereby increasing the male’s fitness.

Heubel and Schlupp, 2008 Figure 2

Heubel and Schlupp, 2008 Firgure 1

Not only do males’ preferences change by season, sailfin
males further mitigate decreased fitness by sperm alterations. When males were
exposed to Amazon mollies, they primed less sperm than males exposed to
conspecifics, or females of their own species (Aspbury and Gabor, 2004). So
sailfin males decreased their sperm viability in the presence of Amazon mollies
in order to prevent unnecessary expenditure of energy. So not only do males
ditch Amazon mollies during the sailfin breeding season, they also don’t spend
as much energy on mating with Amazons as they do with sailfins… men right? Well
really what can we expect when some females only use them for their sperm
casing?

http://www.runningoffthereeses.com/2011/03/

Even though he’s not that into Amazonians, he, being sailfin
mollies, still benefits from mating with them. Females witnessing a male mating
become more ‘attracted’ to him, even if that male is mating with an individual
of another species. So a female sailfin prefers a mating male sailfin. This
preference persists even if that male is mating with an Amazon molly (Schlupp
et al. 1994). Mating, in general, will
increase a male sailfin molly’s chance of perpetuating his genetic material
because he will be more likely to mate with other females that have observed
him mating. Perhaps this is why male mollies haven’t evolved to be completely
discriminating in heterospecific mate choices. Even if he is mating with a
heterospecific that will produce no genetically similar offspring, the chances
of him mating with a conspecific will increase. A mating with a conspecific
will increase his fitness so long as progeny is produced.

What could possibly further this behavior corresponds to
other males witnessing a mating. Sailfin males copy other male’s mate choices.
So if a male sailfin observed another male sailfin mating with an Amazon molly,
he is more likely to mate with an Amazon molly was well (Schlupp and Ryan,
1997). It appears there is a positive feedback loop to mate with
heterospecifics once an individual has mated with an individual of the opposite
sex. This is at least partially the reason why this behavior hasn't been
selected against.

It appears the pros of mating with a heterospecific outweigh
the cons. Which is important in maintaining the Amazon molly species. Ultimately,
Amazon mollies rely on closely related mollies to ensure the species remains
extant. If males weren’t as discriminating against Amazon mollies and in favor
of their corresponding conspecifics, Amazon mollies would go extinct. If male
sailfin mollies chose to mate with Amazon mollies and female sailfin mollies at
the same proportions, Amazon mollies would become extinct (Heubel et al.,
2009). Luckily for the Amazon mollies, males of the sailfin molly species aren’t
so choosy. Otherwise that would mean Amazon mollies would go extinct by sex, or
lack there of. Who would have thought death by sex is applicable in the natural
world.